Alternate-polarization (APol) modulation of optical signals, in which adjacent bits are orthogonally polarized, can significantly increase system tolerance to fiber nonlinearities in pseudo-linear transmission. APol return-to-zero (RZ) differential-phase-shift-keying (DPSK) has been shown to have the highest tolerance to fiber nonlinearities and is considered a promising candidate for 40-Gb/s submarine systems. Compared to a single polarization signal, however, an APol signal has a reduced tolerance to polarization mode dispersion (PMD). For an APol signal, there is no input state of polarization (SOP) that does not cause at least some distortion by even first-order PMD. Moreover, PMD impairments are almost independent of input SOP. To increase the PMD tolerance of an APol signal, PMD compensation is employed. A PMD compensator (PMDC) is typically provided at an optical receiver and is controlled in a feedback arrangement in accordance with a feedback signal generated by some means for detecting PMD.
Some widely used PMDC feedback signals, such as degree of polarization (DOP) and RF tones, do not work for an APol signal. The DOP of an APol signal is close to zero even when there is no PMD, and the relationship of RF tones to PMD is different for different input SOPs. Although eye-opening monitoring and bit error rate (BER) after forward error correction (FEC) can be used as feedback, such arrangements require a fully functional receiver and are not cost effective for a standalone PMDC. Moreover, the speed of such arrangements is limited as well.
Some PMD compensation arrangements require a polarization scrambler at the transmitter. (See, e.g., H. Rosenfeldt et al., “Automatic PMD Compensation at 40 Gbit/s and 80 Gbit/s Using a 3-Dimensional DOP Evaluation for Feedback,” Optical Fiber Communication Conference, 2001.) The use of a polarization scrambler, however, introduces cost, reliability and complexity issues.